1. Field of the Invention
This invention relates, in general, to reclaiming used oil and, more particularly, the invention relates to a hydrogenation process for reprocessing of used oils into secondary raffinates which are in the form of lubricating oils and other byproducts with the addition of hydrogen.
2. Description of the Prior Art
It is known, for example, that in West Germany alone, approximately 500,000 t of used oils are produced each year. In that country, that amount represents approximately 50% of the new oil used in the same time period. It can be assumed that this percentage of used oil to new oil used would be similar in the other major countries throughout the world. If that assumption is correct, it can be seen that vast quantities of used oil are produced each year. The majority of the used oil, at least in West Germany, is processed into so-called secondary raffinates. In the processes presently available, however, u for the processing of used oil into secondary raffinates, they require that the used oil be contaminated only to an extremely small degree with substances which could possibly lead to some degree of environmental damage when the secondary raffinates produced in these presently available processes are reused. The organo-chlorine compounds found in much of the used oils are particularly harmful, in particular PCB (polychlorinated biphenyl). These organo-chlorine contaminants can be the source of the highly-toxic chlorinated dibenzodioxins and dibenzofurane. This is particularly the case if the secondary raffinates containing these contaminates are burned at low temperature. One example of such low temperature burning is the use of the secondary raffinates as a lubricating oil in an internal combustion engine. Polychlorinated biphenyls are used, for example, in cutting oils and hydraulic oils, particularly in the mining industry. Polychlorinated biphenyls are also present in coolant oils that are used in transformers because that makes such coolant oil non-combustible.
It is known in the art that PCB's cannot be microbiologically decomposed. Furthermore, such PCB's are persistent and ubiquitous. Polychlorinated biphenyls have a chemical structure that is similar to DDT (dichlorodiphenyltrichloroethane) and are also found increasingly in the food chain. For example, they are found in fish, milk, etc. Oils that are contaminated in this or in a similar manner and which contain more than 20 mg/kg PCB and/or more than 0.5 wt.% organo-chlorine compounds are considered harmful waste products. Therefore, such contaminated oils are subject to special requirements concerning waste disposal. In the future, it is expected that the permissible limits on these oil contaminates will be reduced even further. Prior to the present invention, used oils with a level of contamination above these values had to be disposed of as special waste. For this purpose, a special waste disposal facility must be provided. An example of such waste disposal facility suitable for this purpose is a high temperature combustion installation. The capacity of such installations presently available in West Germany, for example, is altogether inadequate. Consequently, at the present time, there are bottlenecks encountered in the disposal process. In a high temperature combustion installation, the used oil is burned at temperatures above 1200.degree. C. and generally with hold times of greater than 0.3 seconds, to reduce the production of toxic substances. One example of such toxic substance is dioxin, a known cancer-causing compound.
Even the reprocessing of uncontaminated or only slightly contaminated used oils has been problematic prior to this invention. The sulfuric acid-clay process, for example, which is in itself appropriate for the production of secondary raffinates, also produces an acid resin which is an ecologically objectional residue.
This known disadvantage of the sulfuric acid-clay process was reduced by the KTI process. According to the KTI process, the used oil is first subjected to a physical treatment and thereafter to a so-called hydrofinishing. The physical treatment portion of the process comprises a multi-stage purification process. In a first stage of the purification process, water and easily volatile components are separated from the used oil at atmospheric pressure and at a temperature of 150.degree. C. Then, in a second stage of the purification process, a gas oil fraction is extracted in a vacuum of 20 mbar and a temperature of 270.degree. C. This is followed by a third stage which consists of a high-vacuum flash distillation. Such high-vacuum flash distillation is conducted in a specially designed thin-film evaporator. The thin-film evaporator is operated, at a temperature of 310.degree. C. and a vacuum of 2 mbar, with the use of indirect heat exchange and simultaneous high turbulence in the heat exchange zones. In the KTI process, heavy components of the used oil are extracted as an asphalt-like residue. Such heavy components in the used oil generally include additives, metals and decay products. The fourth stage is the hydro- finishing. In such hydrofinishing state, the only contaminants contained in the purified oil fraction are those contaminated which have the same boiling range as the purified oil fraction itself. These contaminants primarily include components which contain chlorine, oxygen and nitrogen. The hydrofinishing stage of the KTI process is conducted at pressures of approximately 50 bar and temperatures between 300.degree. and 350.degree. C. In this manner, the impurities are extracted as ammonium chloride, gas oil and water. In addition, any sulfur impurities which may be present in the used oil would be removed as H.sub.2 S. The presence of organo-chlorine compounds or ethylene glycol (from antifreeze), however, has a negative impact on the process. The reason being that these substances increase the hydrogen consumption, contaminate the catalyst more rapidly, and also require the use of special catalyst.